1. Field of the Invention
The present invention relates to a magneto-optical recording medium, and more in particular to a magneto-optical recording medium in which data is magnetically recorded by heating the recording medium up to an increased temperature with use of laser beams and from which the recorded information is reproduced by utilizing the magneto-optical effect thereof.
2. Description of the Prior Art
A basic arrangement of a conventional magneto-optical recording medium is as shown in FIGS. 9 and 10. More specifically, on a substrate 71, a perpendicular magnetization film having a magneto-optical effect is formed as a recording film 73 with a protective film 72 interposed therebetween. Further, a protective film 74 is provided on the recording film 73 directly thereon or with a reflective film 75 interposed between the recording film 73 and the protective film 74.
The recording and erasing operation of data on the magneto-optical recording medium is effected in such a way that the recording film 73 is locally heated by irradiation of a laser beam up to a temperature which is higher than the compensation temperature thereof at which the coercive force is made small or over around the Curie temperature to reduce the coercive force of the irradiated portion with a laser beam of the recording film 73 so that the irradiated portion of the recording film 73 is magnetized in the direction of the external magnetic field, which is so-called thermomagnetic recording.
On the other hand, the reproduction of the recorded data is carried out in such a way that a laser beam lower in power than that in recording and erasing operation is irradiated to a portion of the recording medium to rotate a polarization plane of the reflected light or transmitted light of the irradiated laser beam according to the recording state, i.e. direction of magnetization of the recording film 73, which the rotation of the polarization plane takes place due to a magneto-optical effect, so called Kerr effect or Faraday effect. The rotation of the polarization plane is detected as a variation in light intensity by using an analyzer.
Furthermore, with regard to magnetic materials of the recording film 73, in order to implement high-density recording by reducing the interference between the magnetization in opposite directions, there is used a magnetic material having a perpendicular magnetic anisotropy as the recording film 73 for the magneto-optical recording medium.
Conventionally, as a recording film for the magneto-optical recording medium, there have been available some types of films: (1) rare-earth-transition-metal alloy magnetic films, such as TbFeCo; (2) Mn alloy magnetic films, such as MnBi; (3) oxide magnetic films, such as YIG, and the like.
Among such conventional recording films for the magneto-optical recording medium, when using the films of type (1), although the type (1) films are amorphous and therefore there are such advantages that the films are free of noise generation due to crystal grain boundaries during reproduction and there can be rather readily obtained perpendicular magnetic anisotropy films, yet there are problems of inferiority in resistances to oxidation and corrosion and having not so substantially great magneto-optical effect. Regarding these problems, in order to improve the oxidation and corrosion resistances, there has been taken such means that the recording film is protected with protective films 72 and 74 formed of an oxide film such as SiO.sub.2, or a nitride film such as SiN, as shown in FIG. 9. Also, in order to make up for the lack of magneto-optical effect, the film thickness of the protective film 72 in the incident side of reproducing light is so selected that the quantity of reflected light becomes minimum in relation to its refractive index, thereby enhancing the apparent Kerr effect. Alternatively, as shown in FIG. 10, a reflective film 75 is provided behind the recording film 73 having such a thickness as to transmit the reproducing light to some extent, thereby forming an arrangement that takes advantages of the Faraday effect as well as Kerr effect of the recording film 73. These measures, however, do not suffice for the magneto-optical effect. In particular, since the Kerr rotation angle decreases as the wavelength of light is made shorter, it is impossible to implement higher-density recording by use of shorter-wavelength laser beams.
On the other hand, the films of types (2) and (3), while there is an advantage that some are relatively great in their magneto-optical effect, some are involved with a problem that, because of crystalline films, there exhibits characteristic deterioration caused by generation of reproducing noise due to crystal grain boundaries, or by their thermal instability. These types of films also have a problem that, in order to obtain a perpendicular magnetic anisotropy film, it is necessary to heat a substrate at high temperature during film formation or a heat treatment process at a high temperature after film formation, resulting in a disadvantage that a plastic substrate is inapplicable. In particular, when preparing a perpendicular magnetic anisotropy film of a type (3) oxide magnetic film, it is necessary for the substrate to be heated or to be subjected to heat treatment at a high temperature of 500.degree. C. or higher. Further, since the type (3) of oxide magnetic films are nearly transparent, the thickness of the recording film 73 can be increased even when the arrangement of FIG. 10 is adopted. Accordingly, their apparent magneto-optical effect can be increased, but their magneto-optical effect depends largely upon the wavelengths, resulting in a very narrow range of wavelengths at which the magneto-optical effect is substantially great, as low as 100 nm or less. In consequence, the above-mentioned conventional recording films cannot keep up with a large Kerr rotation angle when the wavelengths of laser beams is shortened by degrees as the laser technique progresses for high density recording.